The invention is concerned with a polymeric composition for friction elements having remarkable properties with regard to temperature and contact with water. Although the invention will be described in more details with relation to brake pad or brake linings, it should be understood that it may be used in any application in which friction properties have to remain stable with increasing temperature and with water, such brakes and clutches for vehicles and machine tools. The brake pad is one example in which heat and water are of a prime importance due to a possible overheating if the braking action is applied for an extended period of time, during which moreover water may come In contact with the pads.
Preparations or compositions for friction elements for use in brake pads and other applications are known. One example a is mixture in which a phenolic resin and an organopolysiloxane or silicon resin are mixed with a crosslinking agent and described for instance in EP-0 456 490 and JP-63-251 452.
However, according to IR analysis, this mixture appears to be basically a simple mixture of the original phenolic resin and the product of the homoreaction between the silicon resin and itself. This means in particular that the reaction involved do not lead to specific interactions of the phenolic hydroxy groups with the silicon, most of the phenolic groups remaining as such, i.e as free phenolic groups. Hydrophilic properties are therefore retained together with a relatively high capacity of water absorption, which in turn is affecting strongly the friction characteristics of the product.
The object of the invention is therefore to make the reaction between a phenolic resin and an organopolysiloxane or silicon resin follow a different way, resulting in a actual co-reaction or condensation by co-polymerisation between the phenolic groups and the silanols groups of the silicon in SIxe2x80x94Oxe2x80x94C and Cxe2x80x94Oxe2x80x94C bonds. A part at least of the free phenolic groups of the starting phenolic are consumed In such bonds and will not longer be available for water absorption. The reaction product will loose its hydrophilic properties and the water which may come in contact with said product will not be absorbed, yielding a composition with improved friction properties even under wet conditions.
Another object of the invention is to prepare a composition with superior heat resistance.
Another object of the invention is to prepare a composition with improved wet conditions performance.
In other words, the invention relates to a polymeric composition for friction elements, comprising a co-polymer between (I) a resin containing phenolic groups and a reticulation agent and (II) an organopolysiloxane resin or silicon containing terminal silanol groups, a part of the phenolic groups being bound to the terminal silanol groups.
Preferably, the resin containing phenolic groups is from 50 to 80% and the organopolysiloxane resin containing terminal silanol groups is from 8 to 25% by weight of the total starting mixture.
The starting resin comprising phenolic groups may also comprise terminal non aromatic alcoholic groups, a part at least of the terminal non aromatic alcoholic groups being also bound to the terminal silanol groups.
The reticulation agent may be an amine, such as an hexamine.
In one embodiment of the invention, the reticulation agent is an hexamine and is already present as a mix in a resin containing phenolic groups. Such a starting material is for instance that sold under the name of (copyright)Xylox by Mitsui Toatsu Chemicals. In this commercial product, the resin containing phenolic groups is of the general formula (A) and may Include moieties of a general formula (Axe2x80x2), and contains hexamine (B) in a proportion between 8 and 12% by weight. 
Other starting materials of the same sort may be used as well, such as (copyright)Novalak type of resins
The other compound, namely an organopolysiloxane resin containing terminal silanol groups may be an hydroxy phenyl alkyl silicone resin or methyphenylsiloxane for instance.
The invention relates as well to a process for the preparation of the polymeric composition, comprising the following steps:
a) mixing (I) a resin containing the phenolic groups and the reticulation agent, (II) a resin containing the terminal silanol groups, and (III) an epoxy resin or the epoxidised organopolysiloxane
b) curing the mixture for a period of time sufficient to substantially complete the reaction between the phenolic groups and the terminal silanol groups,
c) post-heating the product obtained under b).
It should be noted that the reaction is made in the presence of an epoxy resin or an epoxidised organopolysiloxane. This will push the reaction towards the way of a condensation or co-reaction leading to a copolymer rather than a simple homoreaction between the silicone resin and itself as mentioned above for the prior art.
Such a reaction involving the epoxy resin may be symbolised as follows: 
The silicone resin is present in the starting mixture from 10 to 20% by weight, preferably around 20%. The epoxy resin may be for instance of (copyright)Ciba-Geigy (GT 7071) type and may be present in the starting mixture from 20 to 40% by weight. The epoxidised organopolysiloxane may be for instance a polydialkylsiloxane and may be present in the starting mixture from 3 to 10%, but preferably around 5%
To make easier the blending of the starting resins, said resins are preferably in a form of powder with a particle size distribution of not more than 400 xcexcm preferably below 300 xcexcm for a compound such as (copyright)Xylok cited above, and 200 xcexcm for silicone.
The mixing step a) which may be held as well as a step for forming or shaping the end product is preferably conducted in a mould at a temperature not exceeding 50xc2x0 C.
In general, the curing step b) is conducted under a pressure of at least 50 atm and a temperature from 80 to 160xc2x0 C. and may be divided in a number of cycles permitting the degassing of the reaction mixture. In this case each degassing cycle is most preferably performed in sequence at increasing pressures and temperatures.
As to the post-heating step c) the temperature is advantageously of at least 200xc2x0 C., under atmospheric pressure.
The various objects and advantages of the invention will become apparent with regard to the following non limitative examples.